We were in a similar position when dh took up marathon-running: 3dc, youngest not yet walking.
It never occurred to me not to support dh!
Together we planned a route where the dc and I could cheer him on as he passed us and then walk to the next 'cheering point'. Something like miles 5, 8, 12 and 25. Between 12 and 25 we went somewhere nice. I forget where we went that first time, but it might have been a coffee shop, library, beach, softplay, museum - we've supported him at many marathons. (Though after the first one we've only cheered him on at beginning and end.)
We had a cheerleading chant for him that the dc loved yelling out, and they really got into clapping and cheering on other runners, too, especially those in fancy dress or struggling.
Then, at the end, dh would stagger stroll up in his space blanket to join us in our agreed meeting place, where we would ply him with hot chocolate, flapjacks, warm jumpers, and ice packs for his knees.